The disposition of gamete and vegetative-cell nuclei in the extending pollen tubes of a grass species, Alopecurus pratensis L

Russell & Cass (1981) have recently reported that in Plumbago zeylandica L. (Plumbaginaceae) the two male gametes and the vegetative nucleus establish a physical association in the pollen grain which is maintained in the extending pollen tube until the time of entry into the embryo sac. The division of the generative cell takes place in the maturing pollen grain in this species, and after the division the gamete cells remain in contact with each other through a common stretch of wall traversed by plasmodesmata, one gamete cell extending into a long, sinuous projection which wraps around, and occupies embayments in, the envelope of the much convoluted vegetative cell nucleus. A similar association of the male gametes with each other and the vegetative nucleus has been described by Dumas & Knox (1984) in another species with tricellular pollen, Brassica oleracea L. (Cruciferae), where one of the male gametes appears actually to be connected in some manner to the envelope of the vegetative nucleus. Russell & Cass (1981) suggest that the connections between the gametes and the consistent association with the vegetative nucleus in P. zeylandica may have some functional importance during the passage through the pollen tube, perhaps in ensuring that the gametes are delivered more or less simultaneously into the embryo sac, which in this species lacks synergids. These observations are obviously of considerable significance, since they may indicate the need for a reassessment of the mechanism of double fertilisation in angiosperms. If something of the nature of a polarised fertilisation-unit is present on the male side, the long held view that the gamete fusions in the embryo sac are non-selective may have to be abandoned. The matter is of some importance for the interpretation of cytoplasmic inheritance through the male line, especially should the two male gametes not be identical one with the other but differ in their organelle content, as Russell & Cass (1981) have shown to be the case in P. zeylandica. Maintained connections between the male gametes during the passage through the tube have been described previously in several genera (e.g., in Vallisneria: Wylie 1941), so this observation cannot be viewed as novel. However, the existence of a persistent linkage with the vegetative nucleus appears not previously to have been reported. Many accounts in the extensive earlier cytological literature show that the vegetative nucleus is often widely separated from the gametes in the pollen tube, and that it may either lead or trail them during extension growth (e.g., Wylie 1923; Poddubnaya-Arnoldi 1936). It would seem, then, that the link between the gametes and the vegetative nucleus must be remarkably elastic, if it is indeed a universal feature.